http://www.cfd-online.com/W/index.php?title=Special:Contributions/Sachinshendge&feed=atom&limit=50&target=Sachinshendge&year=&month=CFD-Wiki - User contributions [en]2016-07-29T20:21:00ZFrom CFD-WikiMediaWiki 1.16.5http://www.cfd-online.com/Wiki/Fluent_FAQFluent FAQ2006-01-04T09:46:52Z<p>Sachinshendge: /* What does the abbreviation mean? */</p>
<hr />
<div>This section is empty. This is just a suggestion on how to structure it. Please feel free to add questions and answers here!<br />
<br />
<br />
== FLUENT ==<br />
=== Solver Related ===<br />
==== What does the floating point error mean? How can I avoid it? ====<br />
<br />
The floating point error has been reported many times and discussed a lot. Here are some of the answers found in the Fluent Forum:<br />
<br />
'''SOLVER AND ITERATION''' -----I think if you set shorter time step, it may be good. Or changing little Under-Relaxiation-Factors, it may be good. In my experience, I set 1/3 Under-Relaxiation-Factors as default.� -----�also lower the values of under relaxation factor and use the coupled implicit solver� -----�Try to change under-relaxation factors and if it is unsteady problem maybe time step is to large.� -----�you can improve the ratio in the solve--control--limits, maybe that can help.� -----�you will need to decrease the Courant number� -----�If you still get the error, initialize the domain with nothing to 'Compute from...' Then click 'init'. Again select the surface from which you want to compute the initial values & iterate. This should work.� -----�Another reason could be a to high courant number - that means, that the steps between two iterations are too large and the change in the results is too large as well (high residuals)�<br />
<br />
'''GRID PROBLEMS''' -----�this error comes when I start scaling grid. in gambit, all my dimension is in mm, when in fluent i convert it in meter using buttone SCALE. after it, when i iterate, about hundred iteration, this error appeared. but when i not scale my drawing to m...and let it be as in gambit..then the iteration is success. -----�hi I think you should check your mesh grid mesh is very high. your problem solve by selection a low mesh.� -----�Your mesh is so heavy that your computers resources are not enough. try to use coarser mesh.�<br />
<br />
'''BOUNDARY CONDITIONS''' -----�In my case I had set a wall boundary condition instead of an axis boundary condition and then FLuent refuses to calculate telling me 'floating point error'.� -----�Your Boudary Conditions do not represent real physis.� -----�wrong boundary condition definition might cause the floating point error. For example setting an internal boundary as interior� -----�Once I had the problem, simulating a 2D chamber with a symmetry BC. I set the symmetry somewhere as �axe symmetric� and the floating point error occur� -----�check the turbulence parameter you set. reduce the turbulence intensity to less that one for first, say 50 iterations.<br />
<br />
'''USING A UDF''' -----�What I mean is really often when people creates UDF they generally forget that for the first iteration some variable can be zero. Therefore if you are divided a number by zero your solver will blow up telling you 'non floating error'. 'non' means 'not a number'. Depending on your UDF this kind of error does not effectively happens at the first iteration. An example, if you are simulated a domain with a stagnant water as initial condition and you are calculated for the first iteration something like 1/Re therefore lets call it BOOM !!! because Re=0 . To find this kind of think there a simple way : reread your UDF.�<br />
<br />
'''MULTI PROCESSOR ISSUES''' -----"I've had similar problems recently with floating point errors on a multi processor simulation. The solution for my problem seems to be to run on a single processor, where it runs fine....?�<br />
<br />
'''WRONG INITIATION''' ----- Initiating the case with wrong conditions may lead to floating point error when the iterations start.<br />
<br />
<br />
<br />
<br />
<br />
<br />
<br />
=== Models Related ===<br />
==== What is the turbulent viscosity ratio warning and how can I handle? ====<br />
<br />
==== How can I determine the inputs for a porous media or porous jump from flow versus pressure drop data? ====<br />
<br />
==== How do I model heat conduction in a composite wall? ====<br />
<br />
==== What pressures should be specified at inlets and outlets for buoyancy flow problems? ====<br />
<br />
==== Are there any general guidelines on selecting a turbulence model? ====<br />
<br />
==== How can both turbulent and laminar flow be included in one model? ====<br />
<br />
==== How to start a 3D simulation with an compressible medium and temperature changes? What is important to consider ====<br />
<br />
<br />
<br />
=== Solution Methodology === <br />
==== How do i carry out rotating body analysis, eg a rotating sphere or cylinder in flow? ====<br />
==== How do i get better and faster convergence? ====<br />
==== What is the role of under-relaxation parameters? What should be the optimum choice of these parameters? ====<br />
They limit the influence of the previous iteration over the present one. If you choose small values it may prevent oscillations in residuum developing. At the same time the solution may need more time to converge. <br />
Keep the default values as they are given in FLUENT. You can decrease them gradually if necessary. Momentum 0.6, pressure 0.1, k 0.4, eps 0.4, mass source 1, viscosity 1.<br />
<br />
<br />
=== Tips === <br />
==== How to merge two mesh files and make one? ====<br />
To merge two mesh files the suggested utility is tmerge. The syntax of tmerge is simple.<br><br />
<i>utility tmerge -3d file1 file2 finalfile </i> <br><br />
To join the two interior faces use: <br><br />
<i>Grid->Fuse</i> <br><br />
from the menu with Fluent.<br />
<br />
==== How to run multiple cases in batch mode ==== <br />
This could be achieved by running it from journal file. The example journal file that runs two cases is given as <br><br />
<i><br />
file read-case-data xxx1.cas <br><br />
solve dual-time-iterate yyy1 <br><br />
file write-case-data zzz1.cas <br><br />
file read-case-data xxx2.cas <br><br />
yes <b>(comment: for discard cas dialog) </b><br><br />
solve dual-time-iterate yyy2 <br><br />
file write-case-data zzz2.cas <br></i><br />
<br />
<br><br />
<br><br />
have a look at this discussion: <br><br />
http://www.cfd-online.com/Forum/fluent_archive.cgi?read=32615<br />
<br />
==== Want to export Fieldview data for postprocssing during iterations ====<br />
This could be done with the help of menu <b>solve->Execute Commands </b>. <br />
Here are two examples: <br><br />
Steady Case <br><br />
<b><br />
:file/export/fug/File_grid-%n <br><br />
:file/export/fud/File_data-%n pressure velocity-magnitude x-velocity y-velocity z-velocity () <br> </b><br />
Unsteady Case <br><br />
<b><br />
:file/export/fug/File_grid-%t <br><br />
:file/export/fud/File_data-%t pressure velocity-magnitude x-velocity y-velocity z-velocity () <br> </b><br />
<br />
You can chose the frequency of export from the <b>Execute Command</b> panel. <br />
<br />
==== What does the abbreviation mean?====<br />
<br />
CFD = Computational Fluid dynamics <br><br />
FEM = Finite element model <br><br />
FVM = Finite Volume Method<br><br />
FDM = Finite Difference Method<br><br />
UDF = User defined function <br><br />
PDF = Probability Density Function<br><br />
URF = Under Relaxation Factor<br><br />
VOF = Volume Of Fluid <br><br />
DPM = Discrete Phase Model <br><br />
TUI = Text User Interface<br><br />
GUI = Graphical user interface<br><br />
BC = Boundary Conditions<br><br />
RANS = Reynolds averaged Navier-Stokes Equations<br><br />
<br />
==== What is the difference between FE and FV?====<br />
<br />
Following excerpt from Vivek Ranade's book will answer this question to some extent. "The distinguishing feature of FE method is that the equation are multiplied by weight function before they are integrated over the entire domain. This approximation is then substituted into the weighted integral of the conservation law. By minimizing the residual, a set of non-linear algebraic equations is obtained. An important advantage of the FE method is its superior ability to deal with a solution domain having complex geometry, It is however difficult to develop computationally efficient solution method for strongly coupled and non-linear equations using FE."<br />
<br><br><br />
" The finite volume (FV) method uses the intergal form of the conservation equation as its starting point. The solution domain is divided into number of computational cells. The differential equation is integrated over the volume of each computational cell to obtain the algebraic equation". Those equations are solved iteratively to get the solution over the entire domain. Even if you start with FV method than also you will get the same form of algebraic equations as with FD method. For most of the cases FV and FD method and FD method is same, only approach is differetnt.<br />
<br />
== FloWizard==<br />
<br />
== FIDAP==<br />
<br />
== POLYFLOW ==<br />
<br />
== Pre-processors ==<br />
<br />
=== Gambit ===<br />
<br />
=== Gambit Turbo ===<br />
<br />
=== TGrid ===<br />
<br />
== Application specific codes ==<br />
<br />
=== Icepak ===<br />
<br />
=== Airpak ===<br />
<br />
=== MixSim ===<br />
<br />
== Educational codes ==<br />
<br />
=== FlowLab ===<br />
<br />
<br />
[[Category: FAQ's]]<br />
<br />
{{stub}}</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2006-01-03T09:52:51Z<p>Sachinshendge: </p>
<hr />
<div>Everyone has always understood that something flows from hot object to cold one. It is called '''''heat'''''. The overall driving force for this heat flow is thermal gradient.<br />
<br />
There are basically three modes of heat transfer:<br />
<br />
1) '''Conduction'''<br />
<br />
2) '''Convection'''<br />
<br />
3) '''Radiation'''<br />
<br />
== Conduction ==<br />
<br />
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. In this mode of heat transfer, heat flow is due to molecular collisions in the substance. Since molecullar collisions increase with temperature (Temperature increases the kinetic energy of the molecules), heat transfer due to conduction increases.<br />
Conduction heat transfer through a substance is because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted (W)}</math><br />
:<math>k = \mbox{Thermal conductivity of the material (W/m K)}</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction (m2)}</math><br />
:<math>T = \mbox{Temperature (K)}</math><br />
:<math>x = \mbox{Length of the object (m)}</math><br />
(-ve sign indicates temperature reduction in heat flow direction)<br />
<br />
== Convection ==<br />
<br />
Convection is heat transfer by means of motion of the molecules in the fluid. Heat energy transfers between a solid and a fluid when there is a temperature difference between the fluid and the solid. Convection heat transfer can not be neglected when there is a significant fluid motion around the solid.<br />
<br />
There are mainly two types of the convection heat transfer:<br />
<br />
1) Natural or Free Convection<br />
<br />
2) Forced Convection<br />
<br />
* '''Natural or Free Convection''':- The temperature of the solid due to an external field such as fluid buoyancy can induce a fluid motion. This is known as "natural convection" and it is a strong function of the temperature difference between the solid and the fluid. This type of convective heat transfer takes place due to only fluid buoyancy caused due to temperature defference between fluid layers.<br />
<br />
*'''Forced Convection''':- Forcing air to blow over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection". Some external means for fluid motion is necessary in this type of convective heat transfer.<br />
<br />
<br />
Fluid mechanics plays a major role in determining convection heat transfer. For each kind of convection heat transfer, the fluid flow can be either laminar or turbulent.<br />
For laminar flow of fluid over solid surface, a steady boundary layer formation takes place through which conductive heat transfer occur. This reduces convective heat transfer rate.<br />
Turbulent flow forms when the boundary layer is shedding or breaking due to higher velocities or rough geometries. This enhances the heat transfer.<br />
<br />
'''Newton's Law of Cooling'''<br />
<br />
Heat transfer due to convection is described by Newton's Law of Cooling, <br />
<br />
:<math>Q_{Convection} = h*A*dT</math><br />
<br />
Where<br />
:<math>Q_{Convection} = \mbox{Heat convected to surrounding fluid (W)}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>A = \mbox{Area of the solid in contact with fluid (m2)}</math><br />
:<math>dT = \mbox{Temperature difference between solid and surrounding fluid (Ts-Tf) (K)}</math><br />
<br />
<br />
The rate of heat transfered to the surrounding fluid is proportional to the object's exposed area A, and the difference between the solid temperature Ts and the mean fluid temperature Tf. Velocity of the fluid over solid is also a major contributing to enhance the rate of heat transfer.<br />
<br />
Convection heat-transfer coefficient (h) plays main role in heat transfer by convection. Heat transfer rates by convection are expressed in terms of h.<br />
<br />
It depends on following factors (h is directly propotional to these factors):<br />
<br />
1) Exposed area of solid<br />
<br />
2) Temperature difference between solid and fluid<br />
<br />
3) Fluid velocity<br />
<br />
Convective heat transfer can also be characterised in terms of Nusselt number.<br />
<br />
Nusselt number is dimensionless number which is useful for heat transfer calculations. Nusselt number is the dimensionless heat transfer coefficient and appears when you are dealing with convection. It, therefore, provides a measure of the convection heat transfer at the surface.<br />
<br />
It can be defined as follows:<br />
<br />
:<math>Nu = \frac{h*l}{k}</math><br />
<br />
Where<br />
:<math>Nu = \mbox{Nusselt number}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>l = \mbox{Characteristic dimension of th esolid object (m)}</math><br />
:<math>k = \mbox{Thermal conductivity of the solid (W/m k)}</math><br />
<br />
The Nusselt number may be viewed as the ratio of heat flow by convection to conduction for a layer of fluid. <br />
<br />
If Nu=1, we have pure conduction. Higher values of Nusselt mean that the heat transfer is enhanced by convection.<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Prandtl_numberPrandtl number2005-12-20T06:03:50Z<p>Sachinshendge: </p>
<hr />
<div>The Prandtl number is defined as<br />
<br />
:<math><br />
Pr = \frac{\mu C_p}{k}<br />
</math><br />
<br />
where<br />
<br />
* <math>\mu</math> is the dynamic viscosity coefficient<br />
* <math>C_p</math> is the specific heat at constant pressure<br />
* <math>k</math> is the coefficient of thermal conduction<br />
<br />
<br />
It is the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity. It can be related to the thickness of the thermal and velocity boundary layers. It is actually the ratio of velocity boundary layer to thermal boundary layer. When Pr=1, the boundary layers coincide. When Pr is small, it means that heat diffuses very quickly compared to the velocity (momentum). This means the thickness of the thermal boundary layer is much bigger than the velocity boundary layer for liquid metals. <br />
<br />
<br />
<br />
[[Category: Dimensionless parameters]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-20T05:55:03Z<p>Sachinshendge: /* Convection */</p>
<hr />
<div>== Conduction ==<br />
<br />
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. In this mode of heat transfer, heat flow is due to molecular collisions in the substance. Since molecullar collisions increase with temperature (Temperature increases the kinetic energy of the molecules), heat transfer due to conduction increases.<br />
Conduction heat transfer through a substance is because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted (W)}</math><br />
:<math>k = \mbox{Thermal conductivity of the material (W/m K)}</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction (m2)}</math><br />
:<math>T = \mbox{Temperature (K)}</math><br />
:<math>x = \mbox{Length of the object (m)}</math><br />
<br />
== Convection ==<br />
<br />
Convection is heat transfer by means of motion of the molecules in the fluid. Heat energy transfers between a solid and a fluid when there is a temperature difference between the fluid and the solid. Convection heat transfer can not be neglected when there is a significant fluid motion around the solid.<br />
<br />
There are mainly two types of the convection heat transfer:<br />
<br />
1) Natural or Free Convection<br />
<br />
2) Forced Convection<br />
<br />
* '''Natural or Free Convection''':- The temperature of the solid due to an external field such as fluid buoyancy can induce a fluid motion. This is known as "natural convection" and it is a strong function of the temperature difference between the solid and the fluid. This type of convective heat transfer takes place due to only fluid buoyancy caused due to temperature defference between fluid layers.<br />
<br />
*'''Forced Convection''':- Forcing air to blow over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection". Some external means for fluid motion is necessary in this type of convective heat transfer.<br />
<br />
<br />
Fluid mechanics plays a major role in determining convection heat transfer. For each kind of convection heat transfer, the fluid flow can be either laminar or turbulent.<br />
For laminar flow of fluid over solid surface, a steady boundary layer formation takes place through which conductive heat transfer occur. This reduces convective heat transfer rate.<br />
Turbulent flow forms when the boundary layer is shedding or breaking due to higher velocities or rough geometries. This enhances the heat transfer.<br />
<br />
'''Newton's Law of Cooling'''<br />
<br />
Heat transfer due to convection is described by Newton's Law of Cooling, <br />
<br />
:<math>Q_{Convection} = h*A*dT</math><br />
<br />
Where<br />
:<math>Q_{Convection} = \mbox{Heat convected to surrounding fluid (W)}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>A = \mbox{Area of the solid in contact with fluid (m2)}</math><br />
:<math>dT = \mbox{Temperature difference between solid and surrounding fluid (Ts-Tf) (K)}</math><br />
<br />
<br />
The rate of heat transfered to the surrounding fluid is proportional to the object's exposed area A, and the difference between the solid temperature Ts and the mean fluid temperature Tf. Velocity of the fluid over solid is also a major contributing to enhance the rate of heat transfer.<br />
<br />
Convection heat-transfer coefficient (h) plays main role in heat transfer by convection. Heat transfer rates by convection are expressed in terms of h.<br />
<br />
It depends on following factors (h is directly propotional to these factors):<br />
<br />
1) Exposed area of solid<br />
<br />
2) Temperature difference between solid and fluid<br />
<br />
3) Fluid velocity<br />
<br />
Convective heat transfer can also be characterised in terms of Nusselt number.<br />
<br />
Nusselt number is dimensionless number which is useful for heat transfer calculations. Nusselt number is the dimensionless heat transfer coefficient and appears when you are dealing with convection. It, therefore, provides a measure of the convection heat transfer at the surface.<br />
<br />
It can be defined as follows:<br />
<br />
:<math>Nu = \frac{h*l}{k}</math><br />
<br />
Where<br />
:<math>Nu = \mbox{Nusselt number}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>l = \mbox{Characteristic dimension of th esolid object (m)}</math><br />
:<math>k = \mbox{Thermal conductivity of the solid (W/m k)}</math><br />
<br />
The Nusselt number may be viewed as the ratio of heat flow by convection to conduction for a layer of fluid. <br />
<br />
If Nu=1, we have pure conduction. Higher values of Nusselt mean that the heat transfer is enhanced by convection.<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/User:SachinshendgeUser:Sachinshendge2005-12-12T09:18:22Z<p>Sachinshendge: </p>
<hr />
<div>Hi<br />
I am in the field of CFD since last two years.<br />
My email ID is sachinshendge@rediffmail.com</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-08T12:00:19Z<p>Sachinshendge: /* Convection */</p>
<hr />
<div>== Conduction ==<br />
<br />
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. In this mode of heat transfer, heat flow is due to molecular collisions in the substance. Since molecullar collisions increase with temperature (Temperature increases the kinetic energy of the molecules), heat transfer due to conduction increases.<br />
Conduction heat transfer through a substance is because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted (W)}</math><br />
:<math>k = \mbox{Thermal conductivity of the material (W/m K)}</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction (m2)}</math><br />
:<math>T = \mbox{Temperature (K)}</math><br />
:<math>x = \mbox{Length of the object (m)}</math><br />
<br />
== Convection ==<br />
<br />
Convection is heat transfer by means of motion of the molecules in the fluid. Heat energy transfers between a solid and a fluid when there is a temperature difference between the fluid and the solid. Convection heat transfer can not be neglected when there is a significant fluid motion around the solid.<br />
<br />
There are mainly two types of the convection heat transfer:<br />
<br />
1) Natural or Free Convection<br />
<br />
2) Forced Convection<br />
<br />
* '''Natural or Free Convection''':- The temperature of the solid due to an external field such as fluid buoyancy can induce a fluid motion. This is known as "natural convection" and it is a strong function of the temperature difference between the solid and the fluid. This type of convective heat transfer takes place due to only fluid buoyancy caused due to temperature defference between fluid layers.<br />
<br />
*'''Forced Convection''':- Forcing air to blow over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection". Some external means for fluid motion is necessary in this type of convective heat transfer.<br />
<br />
<br />
Fluid mechanics plays a major role in determining convection heat transfer. For each kind of convection heat transfer, the fluid flow can be either laminar or turbulent.<br />
For laminar flow of fluid over solid surface, a steady boundary layer formation takes place through which conductive heat transfer occur. This reduces convective heat transfer rate.<br />
Turbulent flow forms when the boundary layer is shedding or breaking due to higher velocities or rough geometries. This enhances the heat transfer.<br />
<br />
'''Newton's Law of Cooling'''<br />
<br />
Heat transfer due to convection is described by Newton's Law of Cooling, <br />
<br />
:<math>Q_{Convection} = h*A*dT</math><br />
<br />
Where<br />
:<math>Q_{Convection} = \mbox{Heat convected to surrounding fluid (W)}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>A = \mbox{Area of the solid in contact with fluid (m2)}</math><br />
:<math>dT = \mbox{Temperature difference between solid and surrounding fluid (Ts-Tf) (K)}</math><br />
<br />
<br />
The rate of heat transfered to the surrounding fluid is proportional to the object's exposed area A, and the difference between the solid temperature Ts and the mean fluid temperature Tf. Velocity of the fluid over solid is also a major contributing to enhance the rate of heat transfer.<br />
<br />
Convection heat-transfer coefficient (h) plays main role in heat transfer by convection. Heat transfer rates by convection are expressed in terms of h.<br />
<br />
It depends on following factors (h is directly propotional to these factors):<br />
<br />
1) Exposed area of solid<br />
<br />
2) Temperature difference between solid and fluid<br />
<br />
3) Fluid velocity<br />
<br />
Convective heat transfer can also be characterised in terms of Nusselt number.<br />
<br />
Nusselt number is dimensionless number which is useful for heat transfer calculations. It can be defined as follows:<br />
<br />
:<math>Nu = \frac{h*l}{k}</math><br />
<br />
Where<br />
:<math>Nu = \mbox{Nusselt number}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>l = \mbox{Characteristic dimension of th esolid object (m)}</math><br />
:<math>k = \mbox{Thermal conductivity of the solid (W/m k)}</math><br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Nusselt_numberNusselt number2005-12-08T11:59:48Z<p>Sachinshendge: </p>
<hr />
<div>Nusselt number is dimensionless number which is useful for convective heat transfer calculations. heat transfer can be expressed in terms of Nusselt number. It can be defined as follows:<br />
<br />
:<math>Nu = \frac{h*l}{k}</math><br />
<br />
Where<br />
:<math>Nu = \mbox{Nusselt number}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>l = \mbox{Characteristic dimension of th esolid object (m)}</math><br />
:<math>k = \mbox{Thermal conductivity of the solid (W/m k)}</math></div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-08T11:58:05Z<p>Sachinshendge: /* Convection */</p>
<hr />
<div>== Conduction ==<br />
<br />
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. In this mode of heat transfer, heat flow is due to molecular collisions in the substance. Since molecullar collisions increase with temperature (Temperature increases the kinetic energy of the molecules), heat transfer due to conduction increases.<br />
Conduction heat transfer through a substance is because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted (W)}</math><br />
:<math>k = \mbox{Thermal conductivity of the material (W/m K)}</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction (m2)}</math><br />
:<math>T = \mbox{Temperature (K)}</math><br />
:<math>x = \mbox{Length of the object (m)}</math><br />
<br />
== Convection ==<br />
<br />
Convection is heat transfer by means of motion of the molecules in the fluid. Heat energy transfers between a solid and a fluid when there is a temperature difference between the fluid and the solid. Convection heat transfer can not be neglected when there is a significant fluid motion around the solid.<br />
<br />
There are mainly two types of the convection heat transfer:<br />
<br />
1) Natural or Free Convection<br />
<br />
2) Forced Convection<br />
<br />
* '''Natural or Free Convection''':- The temperature of the solid due to an external field such as fluid buoyancy can induce a fluid motion. This is known as "natural convection" and it is a strong function of the temperature difference between the solid and the fluid. This type of convective heat transfer takes place due to only fluid buoyancy caused due to temperature defference between fluid layers.<br />
<br />
*'''Forced Convection''':- Forcing air to blow over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection". Some external means for fluid motion is necessary in this type of convective heat transfer.<br />
<br />
<br />
Fluid mechanics plays a major role in determining convection heat transfer. For each kind of convection heat transfer, the fluid flow can be either laminar or turbulent.<br />
For laminar flow of fluid over solid surface, a steady boundary layer formation takes place through which conductive heat transfer occur. This reduces convective heat transfer rate.<br />
Turbulent flow forms when the boundary layer is shedding or breaking due to higher velocities or rough geometries. This enhances the heat transfer.<br />
<br />
'''Newton's Law of Cooling'''<br />
<br />
Heat transfer due to convection is described by Newton's Law of Cooling, <br />
<br />
:<math>Q_{Convection} = h*A*dT</math><br />
<br />
Where<br />
:<math>Q_{Convection} = \mbox{Heat convected to surrounding fluid (W)}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>A = \mbox{Area of the solid in contact with fluid (m2)}</math><br />
:<math>dT = \mbox{Temperature difference between solid and surrounding fluid (Ts-Tf) (K)}</math><br />
<br />
<br />
The rate of heat transfered to the surrounding fluid is proportional to the object's exposed area A, and the difference between the solid temperature Ts and the mean fluid temperature Tf. Velocity of the fluid over solid is also a major contributing to enhance the rate of heat transfer.<br />
<br />
Convection heat-transfer coefficient (h) plays main role in heat transfer by convection. Heat transfer rates by convection are expressed in terms of h.<br />
<br />
It depends on following factors (h is directly propotional to these factors):<br />
<br />
1) Exposed area of solid<br />
<br />
2) Temperature difference between solid and fluid<br />
<br />
3) Fluid velocity<br />
<br />
Convective heat transfer can also be characterised in terms of Nusselt number.<br />
<br />
Nusselt number is dimensionless number which is useful for convective heat transfer calculations. heat transfer can be expressed in terms of Nusselt number. it can be defied as follows:<br />
<br />
:<math>Nu = \frac{h*l}{k}</math><br />
<br />
Where<br />
:<math>Nu = \mbox{Nusselt number}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>l = \mbox{Characteristic dimension of th esolid object (m)}</math><br />
:<math>k = \mbox{Thermal conductivity of the solid (W/m k)}</math><br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/CFD-Wiki:Community_portalCFD-Wiki:Community portal2005-12-08T11:50:31Z<p>Sachinshendge: /* CFD-Wikians - Who we are */</p>
<hr />
<div>This section is intended for people who work on adding content to the Wiki. So fellow CFD-Wikians, this is your page, private hideout, coffee room, coordination center, after-hours bar or whatever you want to use it for. If you still haven't contributed to the Wiki [[CFD-Wiki:Contribute something today|please do so today]]! We need your help and everyone is welcome to join our team of Wiki authors. <br />
<br />
==What's in the works==<br />
<br />
You who do significant additions to the Wiki, please add some information about your work, plans and progress here so that others can see what you are working on and perhaps help, monitor, come with suggestions and most importantly, be inspired by.<br />
<br />
* The CFD-Wiki will be publicly launched on Nov 20, 2005. Before the launch I will focus mainly on improving the administrative sections like content policies, help pages etc. The public launch will be a high-profile event and I'll spare no efforts to get the word out about the faboulous free CFD reference that we have started to create here. I hope that others can help with the rest of the content - it is very important that the CFD-Wiki looks good when we launch it publicly. Surfer are a very picky breed and they will only return and contribute things if they like what they see the first time they visit! --[[User:Jola|Jola]] 07:26, 11 November 2005 (MST)<br />
<br />
* The [[Numerical methods|numerics section]] is groving very quickly now and has a very ambitious table of content. Michail and zxaar are working hard on it. It is a very large area though so if you can help please do so. --[[User:Jola|Jola]] 10:44, 18 September 2005 (MDT)<br />
<br />
* We have been allowed to base the [[Turbulence|turbulence section]] on an excellent book on turbulence written by Professor William K. George. Pavitran is responsible for this work. Expect to see significant additions in this section. --[[User:Jola|Jola]] 10:44, 18 September 2005 (MDT)<br />
<br />
* The [[Validation and test cases|validation and test-case section]] has gotten off to a flying start. Both Praveen and Jasond have already added several cases. --[[User:Jola|Jola]] 10:44, 18 September 2005 (MDT)<br />
<br />
* We have a first [[Special topics|special topics section]] on [[Combustion|combustion]]. Salva and ForMat have created a basic structure and are adding content to it. --[[User:Jola|Jola]] 07:26, 11 November 2005 (MST)<br />
<br />
* I've started work on creating a first best-practise guide. We need some sort of first example for how one of these should look. I chose to start with a guide for [[Best practise guidelines for turbomachinery CFD | turbomachinery CFD]]. --[[User:Jola|Jola]] 10:44, 18 September 2005 (MDT)<br />
<br />
== What needs to be done ==<br />
<br />
''Anything that you want!'' Be bold and just pick something that you feel that you can improve! If you need some help with good ideas on things to work on here are a few suggestions:<br />
<br />
* We have many turbulence models listed in the [[turbulence modeling]] section which still lack any description. Feel free to pick a model that you are familiar with and write a description of it. --[[User:Jola|Jola]] 01:50, 13 September 2005 (MDT)<br />
<br />
* The numerics section<br />
* The [[FAQ's | FAQ]] section is still very thin. If you are familiar with one of the larger CFD codes please consider adding a few questions and answers to the FAQ. --[[User:Jola|Jola]] 08:28, 13 September 2005 (MDT)<br />
<br />
* If you are an experienced CFD engineer and an expert in a special application area you are very welcome to start a [[Best practise guidelines|best practise guideline]] for your speciality. --[[User:Jola|Jola]] 10:44, 18 September 2005 (MDT)<br />
<br />
* ''... add your suggestions on what should be done here''<br />
<br />
* I supose we need to develop strategy of making Wiki. I offer to use the contents of the main CFD-books. It will make easier to start for others -- Michail 19:32, 14 October 2005 (MDT)<br />
<br />
<br />
==Other resources of interest==<br />
<br />
Here are a few links to pages that are of special interest for us CFD-Wikians:<br />
<br />
*[http://www.cfd-online.com/Forum/wiki.cgi Wiki Discussion Forum]<br />
*[[CFD-Wiki:FAQ| CFD-Wiki FAQ]]<br />
<br />
== CFD-Wikians - Who we are ==<br />
<br />
Add your name here if you make contributions to the wiki. The order is alphabetical based on the last name.<br />
<br />
* [[User:ABeevers]] - Adam Beevers<br />
* [[User:praveen]] - Praveen. C<br />
* [[User:Ben]] - Ben D.<br />
* [[User:jasond]] - Jason D.<br />
* [[User:Pavitran]] - Pavitran. D<br />
* [[User:ForMat]] - Matej Forman<br />
* [[User:ganesh]] - Ganesh N<br />
* [[User:harish]] - Harish Gopalan<br />
* [[User:Michail]] - Michail Kirichkov<br />
* [[User:Discoganya]] - Sujit Kirpekar<br />
* [[User:jola]] - Jonas Larsson<br />
* [[User:anurag]] - Anurag Sharma<br />
* [[User:Suneesh]] - Suneesh S.S.<br />
* [[User:zxaar]] - Arjun Yadav<br />
* [[User:Toschi]] - Federico Toschi<br />
* [[User:Tsaad]] - [http://jedi.knows.it Tony Saad]<br />
* [[user:Sachinshendge]] - Sachin Shendge</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-05T08:39:03Z<p>Sachinshendge: /* Conduction */</p>
<hr />
<div>== Conduction ==<br />
<br />
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. In this mode of heat transfer, heat flow is due to molecular collisions in the substance. Since molecullar collisions increase with temperature (Temperature increases the kinetic energy of the molecules), heat transfer due to conduction increases.<br />
Conduction heat transfer through a substance is because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted (W)}</math><br />
:<math>k = \mbox{Thermal conductivity of the material (W/m K)}</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction (m2)}</math><br />
:<math>T = \mbox{Temperature (K)}</math><br />
:<math>x = \mbox{Length of the object (m)}</math><br />
<br />
== Convection ==<br />
<br />
Convection is heat transfer by means of motion of the molecules in the fluid. Heat energy transfers between a solid and a fluid when there is a temperature difference between the fluid and the solid. Convection heat transfer can not be neglected when there is a significant fluid motion around the solid.<br />
<br />
There are mainly two types of the convection heat transfer:<br />
<br />
1) Natural or Free Convection<br />
<br />
2) Forced Convection<br />
<br />
* '''Natural or Free Convection''':- The temperature of the solid due to an external field such as fluid buoyancy can induce a fluid motion. This is known as "natural convection" and it is a strong function of the temperature difference between the solid and the fluid. This type of convective heat transfer takes place due to only fluid buoyancy caused due to temperature defference between fluid layers.<br />
<br />
*'''Forced Convection''':- Forcing air to blow over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection". Some external means for fluid motion is necessary in this type of convective heat transfer.<br />
<br />
<br />
Fluid mechanics plays a major role in determining convection heat transfer. For each kind of convection heat transfer, the fluid flow can be either laminar or turbulent.<br />
For laminar flow of fluid over solid surface, a steady boundary layer formation takes place through which conductive heat transfer occur. This reduces convective heat transfer rate.<br />
Turbulent flow forms when the boundary layer is shedding or breaking due to higher velocities or rough geometries. This enhances the heat transfer.<br />
<br />
'''Newton's Law of Cooling'''<br />
<br />
Heat transfer due to convection is described by Newton's Law of Cooling, <br />
<br />
:<math>Q_{Convection} = h*A*dT</math><br />
<br />
Where<br />
:<math>Q_{Convection} = \mbox{Heat convected to surrounding fluid (W)}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>A = \mbox{Area of the solid in contact with fluid (m2)}</math><br />
:<math>dT = \mbox{Temperature difference between solid and surrounding fluid (Ts-Tf) (K)}</math><br />
<br />
<br />
The rate of heat transfered to the surrounding fluid is proportional to the object's exposed area A, and the difference between the solid temperature Ts and the mean fluid temperature Tf. Velocity of the fluid over solid is also a major contributing to enhance the rate of heat transfer.<br />
<br />
Convection heat-transfer coefficient (h) plays main role in heat transfer by convection. Heat transfer rates by convection are expressed in terms of h.<br />
<br />
It depends on following factors (h is directly propotional to these factors):<br />
<br />
1) Exposed area of solid<br />
<br />
2) Temperature difference between solid and fluid<br />
<br />
3) Fluid velocity<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Nusselt_numberNusselt number2005-12-05T08:37:22Z<p>Sachinshendge: </p>
<hr />
<div>=='''Nusselt number'''==<br />
<br />
Nusselt number is dimensionless number which is useful for convective heat transfer calculations. heat transfer can be expressed in terms of Nusselt number. it can be defied as follows:<br />
<br />
'''<math>Nu = h*l/k</math>'''<br />
<br />
Where<br />
:<math>Nu = \mbox{Nusselt number}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>l = \mbox{Characteristic dimension of th esolid object (m)}</math><br />
:<math>k = \mbox{Thermal conductivity of the solid (W/m k)}</math></div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-05T08:36:26Z<p>Sachinshendge: /* Convection */</p>
<hr />
<div>== Conduction ==<br />
<br />
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. In this mode of heat transfer, heat flow is due to molecular collisions in the substance. Since molecullar collisions increase with temperature (Temperature increases the kinetic energy of the molecules), heat transfer due to conduction increases.<br />
Conduction heat transfer through a substance is because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted}\;[W]</math><br />
:<math>k = \mbox{Thermal conductivity of the material}\;[W/m\,K]</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction}\;[m^2]</math><br />
:<math>T = \mbox{Temperature}\;[K]</math><br />
:<math>x = \mbox{Length of the object}\;[m]</math><br />
<br />
== Convection ==<br />
<br />
Convection is heat transfer by means of motion of the molecules in the fluid. Heat energy transfers between a solid and a fluid when there is a temperature difference between the fluid and the solid. Convection heat transfer can not be neglected when there is a significant fluid motion around the solid.<br />
<br />
There are mainly two types of the convection heat transfer:<br />
<br />
1) Natural or Free Convection<br />
<br />
2) Forced Convection<br />
<br />
* '''Natural or Free Convection''':- The temperature of the solid due to an external field such as fluid buoyancy can induce a fluid motion. This is known as "natural convection" and it is a strong function of the temperature difference between the solid and the fluid. This type of convective heat transfer takes place due to only fluid buoyancy caused due to temperature defference between fluid layers.<br />
<br />
*'''Forced Convection''':- Forcing air to blow over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection". Some external means for fluid motion is necessary in this type of convective heat transfer.<br />
<br />
<br />
Fluid mechanics plays a major role in determining convection heat transfer. For each kind of convection heat transfer, the fluid flow can be either laminar or turbulent.<br />
For laminar flow of fluid over solid surface, a steady boundary layer formation takes place through which conductive heat transfer occur. This reduces convective heat transfer rate.<br />
Turbulent flow forms when the boundary layer is shedding or breaking due to higher velocities or rough geometries. This enhances the heat transfer.<br />
<br />
'''Newton's Law of Cooling'''<br />
<br />
Heat transfer due to convection is described by Newton's Law of Cooling, <br />
<br />
:<math>Q_{Convection} = h*A*dT</math><br />
<br />
Where<br />
:<math>Q_{Convection} = \mbox{Heat convected to surrounding fluid (W)}</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality ) (W/m2 K)}</math><br />
:<math>A = \mbox{Area of the solid in contact with fluid (m2)}</math><br />
:<math>dT = \mbox{Temperature difference between solid and surrounding fluid (Ts-Tf) (K)}</math><br />
<br />
<br />
The rate of heat transfered to the surrounding fluid is proportional to the object's exposed area A, and the difference between the solid temperature Ts and the mean fluid temperature Tf. Velocity of the fluid over solid is also a major contributing to enhance the rate of heat transfer.<br />
<br />
Convection heat-transfer coefficient (h) plays main role in heat transfer by convection. Heat transfer rates by convection are expressed in terms of h.<br />
<br />
It depends on following factors (h is directly propotional to these factors):<br />
<br />
1) Exposed area of solid<br />
<br />
2) Temperature difference between solid and fluid<br />
<br />
3) Fluid velocity<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-05T08:14:49Z<p>Sachinshendge: /* Convection */</p>
<hr />
<div>== Conduction ==<br />
<br />
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. In this mode of heat transfer, heat flow is due to molecular collisions in the substance. Since molecullar collisions increase with temperature (Temperature increases the kinetic energy of the molecules), heat transfer due to conduction increases.<br />
Conduction heat transfer through a substance is because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted}\;[W]</math><br />
:<math>k = \mbox{Thermal conductivity of the material}\;[W/m\,K]</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction}\;[m^2]</math><br />
:<math>T = \mbox{Temperature}\;[K]</math><br />
:<math>x = \mbox{Length of the object}\;[m]</math><br />
<br />
== Convection ==<br />
<br />
Convection is heat transfer by means of motion of the molecules in the fluid. Heat energy transfers between a solid and a fluid when there is a temperature difference between the fluid and the solid. Convection heat transfer can not be neglected when there is a significant fluid motion around the solid.<br />
<br />
There are mainly two types of the convection heat transfer:<br />
<br />
1) Natural or Free Convection<br />
<br />
2) Forced Convection<br />
<br />
* '''Natural or Free Convection''':- The temperature of the solid due to an external field such as fluid buoyancy can induce a fluid motion. This is known as "natural convection" and it is a strong function of the temperature difference between the solid and the fluid. This type of convective heat transfer takes place due to only fluid buoyancy caused due to temperature defference between fluid layers.<br />
<br />
*'''Forced Convection''':- Forcing air to blow over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection". Some external means for fluid motion is necessary in this type of convective heat transfer.<br />
<br />
<br />
Fluid mechanics plays a major role in determining convection heat transfer. For each kind of convection heat transfer, the fluid flow can be either laminar or turbulent.<br />
For laminar flow of fluid over solid surface, a steady boundary layer formation takes place through which conductive heat transfer occur. This reduces convective heat transfer rate.<br />
Turbulent flow forms when the boundary layer is shedding or breaking due to higher velocities or rough geometries. This enhances the heat transfer.<br />
<br />
'''Newton's Law of Cooling'''<br />
<br />
Heat transfer due to convection is described by Newton's Law of Cooling, <br />
<br />
:<math>Q_{Convection} = h*A*dT</math><br />
<br />
Where<br />
:<math>Q_{Convection} = \mbox{Heat convected to surrounding fluid}\;[W]</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality )}\;[W/m^2\,K]</math><br />
:<math>A = \mbox{Area of the solid in contact with fluid}\;[m^2]</math><br />
:<math>dT = \mbox{Temperature difference between solid and surrounding fluid (Ts-Tf)}\;[K]</math><br />
<br />
<br />
The rate of heat transfered to the surrounding fluid is proportional to the object's exposed area A, and the difference between the solid temperature Ts and the mean fluid temperature Tf. Velocity of the fluid over solid is also a major contributing to enhance the rate of heat transfer.<br />
<br />
Convection heat-transfer coefficient (h) plays main role in heat transfer by convection. Heat transfer rates by convection are expressed in terms of h.<br />
<br />
It depends on following factors (h is directly propotional to these factors):<br />
<br />
1) Exposed area of solid<br />
<br />
2) Temperature difference between solid and fluid<br />
<br />
3) Fluid velocity<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-05T05:23:39Z<p>Sachinshendge: /* Convection */</p>
<hr />
<div>== Conduction ==<br />
<br />
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. In this mode of heat transfer, heat flow is due to molecular collisions in the substance. Since molecullar collisions increase with temperature (Temperature increases the kinetic energy of the molecules), heat transfer due to conduction increases.<br />
Conduction heat transfer through a substance is because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted}\;[W]</math><br />
:<math>k = \mbox{Thermal conductivity of the material}\;[W/m\,K]</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction}\;[m^2]</math><br />
:<math>T = \mbox{Temperature}\;[K]</math><br />
:<math>x = \mbox{Length of the object}\;[m]</math><br />
<br />
== Convection ==<br />
<br />
Conduction is heat transfer by means of motion of the molecules in the fluid. Heat energy transfers between a solid and a fluid when there is a temperature difference between the fluid and the solid. Convection heat transfer can not be neglected when there is a significant fluid motion around the solid.<br />
<br />
There are mainly two types of the convection heat transfer:<br />
<br />
1) Natural or Free Convection<br />
<br />
2) Forced Convection<br />
<br />
* '''Natural or Free Convection''':- The temperature of the solid due to an external field such as fluid buoyancy can induce a fluid motion. This is known as "natural convection" and it is a strong function of the temperature difference between the solid and the fluid. This type of convective heat transfer takes place due to only fluid buoyancy caused due to temperature defference between fluid layers.<br />
<br />
*'''Forced Convection''':- Forcing air to blow over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection". Some external means for fluid motion is necessary in this type of convective heat transfer.<br />
<br />
<br />
Fluid mechanics plays a major role in determining convection heat transfer. For each kind of convection heat transfer, the fluid flow can be either laminar or turbulent.<br />
For laminar flow of fluid over solid surface, a steady boundary layer formation takes place through which conductive heat transfer occur. This reduces convective heat transfer rate.<br />
Turbulent flow forms when the boundary layer is shedding or breaking due to higher velocities or rough geometries. This enhances the heat transfer.<br />
<br />
'''Newton's Law of Cooling'''<br />
<br />
Heat transfer due to convection is described by Newton's Law of Cooling, <br />
<br />
:<math>Q_{Convection} = h*A*dT</math><br />
<br />
Where<br />
:<math>Q_{Convection} = \mbox{Heat convected to surrounding fluid}\;[W]</math><br />
:<math>h = \mbox{Convection heat-transfer coefficient (constant of proportionality )}\;[W/m^2\,K]</math><br />
:<math>A = \mbox{Area of the solid in contact with fluid}\;[m^2]</math><br />
:<math>dT = \mbox{Temperature difference between solid and surrounding fluid (Ts-Tf)}\;[K]</math><br />
<br />
<br />
The rate of heat transfered to the surrounding fluid is proportional to the object's exposed area A, and the difference between the solid temperature Ts and the fluid free-stream temperature Tf.<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-02T10:52:00Z<p>Sachinshendge: /* Conduction */</p>
<hr />
<div>== Conduction ==<br />
<br />
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. In this mode of heat transfer, heat flow is due to molecular collisions in the substance. Since molecullar collisions increase with temperature (Temperature increases the kinetic energy of the molecules), heat transfer due to conduction increases.<br />
Conduction heat transfer through a substance is because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted}\;[W]</math><br />
:<math>k = \mbox{Thermal conductivity of the material}\;[W/m\,K]</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction}\;[m^2]</math><br />
:<math>T = \mbox{Temperature}\;[K]</math><br />
:<math>x = \mbox{Length of the object}\;[m]</math><br />
<br />
== Convection ==<br />
<br />
<br />
<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/NomenclatureNomenclature2005-12-02T09:41:00Z<p>Sachinshendge: /* Dimensionless parameters: */</p>
<hr />
<div><table cellspacing="10"><br />
<tr><br />
<th align="left">Acronym</th><br />
<th align="left">Description</th><br />
<th align="left">Unit</th><br />
</tr><br />
<tr><br />
<td><math>C_p</math></td><br />
<td>Specific heat at constant pressure</td><br />
<td><math>J/kgK</math></td><br />
</tr><br />
<tr><br />
<td><math>C_v</math></td><br />
<td>Specific heat at constant volume</td><br />
<td><math>J/kgK</math></td><br />
</tr><br />
<tr><br />
<td><math>e</math></td><br />
<td>Internal energy</td><br />
<td><math>J/kg</math></td><br />
</tr><br />
<tr><br />
<td><math>e_0</math></td><br />
<td>Total energy</td><br />
<td><math>J/kg</math></td><br />
</tr><br />
<tr><br />
<td><math>h</math></td><br />
<td>Enthalpy</td><br />
<td><math>J/kg</math></td><br />
</tr><br />
<tr><br />
<td><math>k</math></td><br />
<td>Turbulent kinetic energy</td><br />
<td><math>J/kg</math></td><br />
</tr><br />
<tr><br />
<td><math>p</math></td><br />
<td>Static pressure</td><br />
<td><math>Pa</math></td><br />
</tr><br />
<tr><br />
<td><math>R</math></td><br />
<td>Specific gas constant</td><br />
<td><math>J/kgK</math></td><br />
</tr><br />
<tr><br />
<td><math>S_{ij}^*</math></td><br />
<td>Trace-less viscous strain-rate tensor</td><br />
<td><math>s^{-1}</math></td><br />
</tr><br />
<tr><br />
<td><math>t</math></td><br />
<td>Time</td><br />
<td><math>s</math></td><br />
</tr><br />
<tr><br />
<td><math>T</math></td><br />
<td>Static temperature</td><br />
<td><math>K</math></td><br />
</tr><br />
<tr><br />
<td><math>u^*</math></td><br />
<td>Friction velocity</td><br />
<td><math>m/s</math></td><br />
</tr><br />
<tr><br />
<td><math>u_i</math></td><br />
<td>Velocity</td><br />
<td><math>m/s</math></td><br />
</tr><br />
<tr><br />
<td><math>\gamma</math></td><br />
<td>[[Ratio of specific heats | Specific heat ratio]] = <math>C_p/C_v</math></td><br />
<td></td><br />
</tr><br />
<tr><br />
<td><math>\delta_{ij}</math></td><br />
<td>Kronecker's delta function</td><br />
<td></td><br />
</tr><br />
<tr><br />
<td><math>\mu</math></td><br />
<td>Dynamic viscosity</td><br />
<td><math>Ns/m^2</math></td><br />
</tr><br />
<tr><br />
<td><math>\nu</math></td><br />
<td>Kinematic viscosity</td><br />
<td><math>m^2/s</math></td><br />
</tr><br />
<tr><br />
<td><math>\rho</math></td><br />
<td>Density</td><br />
<td><math>kg/m^3</math></td><br />
</tr><br />
<tr><br />
<td><math>\tau_{ij}</math></td><br />
<td>Shear stress tensor</td><br />
<td><math>N/m^2</math></td><br />
</tr><br />
<tr><br />
<td><math>\omega</math></td><br />
<td>Specific dissipation</td><br />
<td><math>s^{-1}</math></td><br />
</tr><br />
<br />
<tr><br />
<td><math>Y_k</math></td><br />
<td> [[Mass Fraction]] of species k</td><br />
</tr><br />
<br />
<tr><br />
<td><math>X_k</math></td><br />
<td> [[Molar Fraction]] of species k</td> <br />
</tr><br />
<br />
<tr><br />
<td><math>[X_k]</math></td><br />
<td>Molar Concentration of species k</td><br />
<td><math>mol/m^3</math></td><br />
</tr><br />
<br />
<tr><br />
<td><math>W_k</math></td><br />
<td>Molecular weight of species k</td><br />
<td><math>kg/mol</math></td><br />
</tr><br />
<br />
<tr><br />
<td><math> \overline{W} </math></td><br />
<td>Mean molecular weight of a mixture</td><br />
<td><math>kg/mol</math></td><br />
</tr><br />
<br />
<tr><br />
<td><math> \dot \omega_k </math></td><br />
<td>k-species reaction rate </td><br />
<td><math>kg/m^3 s</math></td><br />
</tr><br />
<br />
<br />
</table><br />
<br />
== Dimensionless parameters: ==<br />
<br />
<table cellspacing="10"><br />
<tr><br />
<th align="left">Parameter</th><br />
<th align="left">Description</th><br />
<th align="left">Definition</th><br />
</tr><br />
<tr><br />
<td><math>Nu</math></td><br />
<td>[[Nusselt number]]</td><br />
<td><math>\frac{h L}{k}</math></td><br />
</tr><br />
<tr><br />
<td><math>Pr</math></td><br />
<td>[[Prandtl number]]</td><br />
<td><math>\frac{\mu C_p}{k}</math></td><br />
</tr><br />
<tr><br />
<td><math>Re</math></td><br />
<td>[[Reynolds number]]</td><br />
<td><math>\frac{\rho VL}{\mu}</math></td><br />
</tr><br />
<tr><br />
<td><math>St</math></td><br />
<td>[[Strouhal number]]</td><br />
<td><math>\frac{fL}{V}</math></td><br />
</tr><br />
<tr><br />
<td><math>Ma</math></td><br />
<td>[[Mach number]]</td><br />
<td><math>\frac{v}{C}</math></td><br />
</tr><br />
</table><br />
<br />
== Subscript: ==<br />
<br />
<table cellspacing="10"><br />
<tr><td><math>t</math></td><td>Turbulent property</td></tr><br />
<tr><td><math>0</math></td><td>Stagnation / total property</td></tr><br />
</table><br />
<br />
== Superscript: ==<br />
<br />
<table cellspacing="10"><br />
<tr><td><math>conv</math></td><td>Convective part</td></tr><br />
<tr><td><math>diff</math></td><td>Diffusive part</td></tr><br />
<tr><td><math>lam</math></td><td>Laminar part</td></tr><br />
<tr><td><math>tot</math></td><td>Laminar + turbulent part</td></tr><br />
<tr><td><math>turb</math></td><td>Turbulent part</td></tr><br />
<tr><td><math>'</math></td><td>Reynolds fluctuating part</td></tr><br />
<tr><td><math>''</math></td><td>Favre fluctuating part</td></tr><br />
<tr><td><math>\widetilde{\cdot}</math></td><td>Density weighted (Favre) average</td></tr><br />
<tr><td><math>\overline{\cdot}</math></td><td>Normal average (time or space)</td></tr><br />
</table></div>Sachinshendgehttp://www.cfd-online.com/Wiki/NomenclatureNomenclature2005-12-02T09:38:59Z<p>Sachinshendge: /* Dimensionless parameters: */</p>
<hr />
<div><table cellspacing="10"><br />
<tr><br />
<th align="left">Acronym</th><br />
<th align="left">Description</th><br />
<th align="left">Unit</th><br />
</tr><br />
<tr><br />
<td><math>C_p</math></td><br />
<td>Specific heat at constant pressure</td><br />
<td><math>J/kgK</math></td><br />
</tr><br />
<tr><br />
<td><math>C_v</math></td><br />
<td>Specific heat at constant volume</td><br />
<td><math>J/kgK</math></td><br />
</tr><br />
<tr><br />
<td><math>e</math></td><br />
<td>Internal energy</td><br />
<td><math>J/kg</math></td><br />
</tr><br />
<tr><br />
<td><math>e_0</math></td><br />
<td>Total energy</td><br />
<td><math>J/kg</math></td><br />
</tr><br />
<tr><br />
<td><math>h</math></td><br />
<td>Enthalpy</td><br />
<td><math>J/kg</math></td><br />
</tr><br />
<tr><br />
<td><math>k</math></td><br />
<td>Turbulent kinetic energy</td><br />
<td><math>J/kg</math></td><br />
</tr><br />
<tr><br />
<td><math>p</math></td><br />
<td>Static pressure</td><br />
<td><math>Pa</math></td><br />
</tr><br />
<tr><br />
<td><math>R</math></td><br />
<td>Specific gas constant</td><br />
<td><math>J/kgK</math></td><br />
</tr><br />
<tr><br />
<td><math>S_{ij}^*</math></td><br />
<td>Trace-less viscous strain-rate tensor</td><br />
<td><math>s^{-1}</math></td><br />
</tr><br />
<tr><br />
<td><math>t</math></td><br />
<td>Time</td><br />
<td><math>s</math></td><br />
</tr><br />
<tr><br />
<td><math>T</math></td><br />
<td>Static temperature</td><br />
<td><math>K</math></td><br />
</tr><br />
<tr><br />
<td><math>u^*</math></td><br />
<td>Friction velocity</td><br />
<td><math>m/s</math></td><br />
</tr><br />
<tr><br />
<td><math>u_i</math></td><br />
<td>Velocity</td><br />
<td><math>m/s</math></td><br />
</tr><br />
<tr><br />
<td><math>\gamma</math></td><br />
<td>[[Ratio of specific heats | Specific heat ratio]] = <math>C_p/C_v</math></td><br />
<td></td><br />
</tr><br />
<tr><br />
<td><math>\delta_{ij}</math></td><br />
<td>Kronecker's delta function</td><br />
<td></td><br />
</tr><br />
<tr><br />
<td><math>\mu</math></td><br />
<td>Dynamic viscosity</td><br />
<td><math>Ns/m^2</math></td><br />
</tr><br />
<tr><br />
<td><math>\nu</math></td><br />
<td>Kinematic viscosity</td><br />
<td><math>m^2/s</math></td><br />
</tr><br />
<tr><br />
<td><math>\rho</math></td><br />
<td>Density</td><br />
<td><math>kg/m^3</math></td><br />
</tr><br />
<tr><br />
<td><math>\tau_{ij}</math></td><br />
<td>Shear stress tensor</td><br />
<td><math>N/m^2</math></td><br />
</tr><br />
<tr><br />
<td><math>\omega</math></td><br />
<td>Specific dissipation</td><br />
<td><math>s^{-1}</math></td><br />
</tr><br />
<br />
<tr><br />
<td><math>Y_k</math></td><br />
<td> [[Mass Fraction]] of species k</td><br />
</tr><br />
<br />
<tr><br />
<td><math>X_k</math></td><br />
<td> [[Molar Fraction]] of species k</td> <br />
</tr><br />
<br />
<tr><br />
<td><math>[X_k]</math></td><br />
<td>Molar Concentration of species k</td><br />
<td><math>mol/m^3</math></td><br />
</tr><br />
<br />
<tr><br />
<td><math>W_k</math></td><br />
<td>Molecular weight of species k</td><br />
<td><math>kg/mol</math></td><br />
</tr><br />
<br />
<tr><br />
<td><math> \overline{W} </math></td><br />
<td>Mean molecular weight of a mixture</td><br />
<td><math>kg/mol</math></td><br />
</tr><br />
<br />
<tr><br />
<td><math> \dot \omega_k </math></td><br />
<td>k-species reaction rate </td><br />
<td><math>kg/m^3 s</math></td><br />
</tr><br />
<br />
<br />
</table><br />
<br />
== Dimensionless parameters: ==<br />
<br />
<table cellspacing="10"><br />
<tr><br />
<th align="left">Parameter</th><br />
<th align="left">Description</th><br />
<th align="left">Definition</th><br />
</tr><br />
<tr><br />
<td><math>Nu</math></td><br />
<td>[[Nusselt number]]</td><br />
<td><math>\frac{h L}{k}</math></td><br />
</tr><br />
<tr><br />
<td><math>Pr</math></td><br />
<td>[[Prandtl number]]</td><br />
<td><math>\frac{\mu C_p}{k}</math></td><br />
</tr><br />
<tr><br />
<td><math>Re</math></td><br />
<td>[[Reynolds number]]</td><br />
<td><math>\frac{\rho VL}{\mu}</math></td><br />
</tr><br />
<tr><br />
<td><math>St</math></td><br />
<td>[[Strouhal number]]</td><br />
<td><math>\frac{fL}{V}</math></td><br />
</tr><br />
</table><br />
<br />
== Subscript: ==<br />
<br />
<table cellspacing="10"><br />
<tr><td><math>t</math></td><td>Turbulent property</td></tr><br />
<tr><td><math>0</math></td><td>Stagnation / total property</td></tr><br />
</table><br />
<br />
== Superscript: ==<br />
<br />
<table cellspacing="10"><br />
<tr><td><math>conv</math></td><td>Convective part</td></tr><br />
<tr><td><math>diff</math></td><td>Diffusive part</td></tr><br />
<tr><td><math>lam</math></td><td>Laminar part</td></tr><br />
<tr><td><math>tot</math></td><td>Laminar + turbulent part</td></tr><br />
<tr><td><math>turb</math></td><td>Turbulent part</td></tr><br />
<tr><td><math>'</math></td><td>Reynolds fluctuating part</td></tr><br />
<tr><td><math>''</math></td><td>Favre fluctuating part</td></tr><br />
<tr><td><math>\widetilde{\cdot}</math></td><td>Density weighted (Favre) average</td></tr><br />
<tr><td><math>\overline{\cdot}</math></td><td>Normal average (time or space)</td></tr><br />
</table></div>Sachinshendgehttp://www.cfd-online.com/Wiki/Best_practice_guidelines_for_turbomachinery_CFDBest practice guidelines for turbomachinery CFD2005-12-02T05:04:15Z<p>Sachinshendge: /* Multi-stage analysis */</p>
<hr />
<div>== Introduction ==<br />
<br />
This guide is mainly aimed at axial turbomachinery. Feel free to extend it to other types of turbomachinery though. <br />
<br />
== Deciding what type of simulation to do ==<br />
<br />
=== 2D, Quasi-3D or 3D ===<br />
<br />
=== Inviscid or viscid ===<br />
<br />
=== Transient or Stationary ===<br />
<br />
== Meshing ==<br />
<br />
In general, for turbomachinery blading simulations a structured grid, especially in the boundary layers, is preferable and will most often provide superior accuracy over an unstructured grid. <br />
<br />
=== Mesh size guidelines ===<br />
<br />
In 3D a decent wall-function mesh typically has around 100,000 cells. This type of mesh size is suitable for quick design iterations where it is not essential to resolve all secondary flows and vortices. A good wall-function mesh intended to resolve secondary flows well should have at least 400,000 cells. A good low-Re mesh with resolved boundary layers typically has around 1,000,000 cells.<br />
<br />
In 2D a good wall-function mesh has around 10,000 and a good low-Re mesh with resolved boundary layers has around 30,000 cells.<br />
<br />
It is important to resolve leading and trailing edges well. Typically at least 10 cells, preferably 20 should be used around the leading and tralining edges. For very blunt and large leading edges, like those commonly found on HP turbine blades, 30 or more cells can be necessary. <br />
<br />
Cases which are difficult to converge with a steady simulation and which show tendencies of periodic vortex shedding from the trailing edge, can sometimes be "tamed" by using a coarse mesh around the trailing edge. This, of course, reduces the accuracy but can be a trick to obtain a converged solution if time and computer resouces does not allow a transient simulation to be performed.<br />
<br />
=== Boundary layer mesh ===<br />
<br />
For design iteration type of simulations where, a [[wall function]] approach is sufficient, [[y-plus | y+]] for the first cell should be somewhere between 30 and 300. For more accurate simulations with resolved boundary layers the mesh should have a y+ for the first cell which is below 1. A good rule of thumb is to use a growth ratio in the boundary layer of 1.2 - 1.25. <br />
<br />
If you are uncertain of which wall distance to mesh with you can use a [http://www.cfd-online.com/Links/tools.html#yplus y+ estimation tool] to estitmate the distance needed to obtain the desired y+.<br />
<br />
As a rule of thumb a wall-function mesh typically requires about 10 cells in the boundary layer whereas a resolved low-Re mesh requires about 40 cells in the boundary layer. <br />
<br />
== Boundary conditions ==<br />
<br />
Describe different types of boundary conditions and when they should be used:<br />
<br />
* Total pressure in, static pressure out<br />
* Absorbing boundary conditions<br />
* ...<br />
<br />
Describe how to select inlet turbulence level and length-scale <br />
<br />
== Turbulence modeling ==<br />
<br />
Selecting a suitable turbulence model for turbomachinery simulations can be a challenging task. There is no single model which is suitable for all types of simulations. Which turbulence model CFD engineers use often has as much to do with beliefs and traditions as with knowledge and facts. There are many diffrent schools. However, below follows some advices that most CFD engineers in the turbomachinery field tend to agree upon.<br />
<br />
For attached flows close to the design point a simple algebraic model like the [[Baldwin-Lomax model]] can be used. Another common choice for design-iteration type of simulations is the one-equation model by [[Spalart-Allmaras model | Spalart-Allmaras]]. The big advantage with both the [[Baldwin-Lomax model]] and the [[Spalart-Allmaras model]] over more advanced models are that they are very robust to use and rarely produce complete unphysical results.<br />
<br />
In order to accurately predict more difficult cases, like flows that are close to or even fully separated, rotating flows, flows strongly affected by secondary flows etc. it is often necessary to use a more refined turbulence model. Common choices are a two-equation models like the <math>k-\epsilon</math>. <math>k-\epsilon</math> models can give good results but this type of models need to include some form of correction to avoíd over-production of turbulent energy in regions with strong acceleration or decelleration. Typical such corrections are some form or [[realizability constraint]] or the [[Kato-Launder modification]]. Antoher common choice in turbulence model is Menter's [[SST k-omega model]] or the slightly more elaborate [[v2f model]] by Durbin.<br />
<br />
=== Near-wall treatment ===<br />
<br />
For on-design simlations without any large separated regions it is often sufficient to use a [[wall-function model]] close to the wall, preferably using some form of non-equilibrium wall-function that is sensitised to streamwise pressure gradients.<br />
<br />
For off-design simulation, or simulations involving complex secondary flows and separations, it is often necessary to use a [[low-Re model]]. There exists many low-Re models that have been used with success in turbomachinery simulations. A robust and often good choice is to use a one-equation model, like for example the [[Wolfstein model]], in the inner parts of the boundary layer. There are also several Low-Re <math>k-\epsilon</math> models that work well. Just make sure they don't suffer from the problem with overproduction of turbulent energy in regions with strong acceleration or decelleration. In the last few years Menter's low-Re <math>SST k-\omega</math> model has gained increased popularity. <br />
<br />
=== Transition prediction ===<br />
<br />
Transition refers to the process when a lamainar boundary layer becomes unstable and transitions to a turbulent boundary layer. There are two types of transition - natural transition, where inherent instabilities in the boundary layer cause the transition and by-pass transition, where convection and diffusion of turbulence from the free-stream into the boundary layer causes transition. <br />
<br />
== Numerical considerations ==<br />
<br />
== Multi-stage analysis ==<br />
<br />
Types of analysis:<br />
<br />
* Frozen rotor<br />
:Frozen rotor defines the domain interface to transfer flow and thermal data across the interface between the stationary and rotating domain. Frozen rotor interface connects the two doamins (rotating and stationary) in such a way that these two domains have a fixed relative position throughout the solution, but with desired frame transformation occurring across the interface. Transient effects cannot be modelled with this interface. This the the only disadvantage of the frozen rotor interface.<br />
* Mixing-plane<br />
* Sliding-mesh<br />
* Time-inclinded<br />
* Adamszyk stresses ...<br />
<br />
''Describe how to scale blade-sections when doing sliding-mesh computations''<br />
<br />
== Heat transfer predictions ==<br />
<br />
== Acoustics and noise ==<br />
<br />
A whole separate research subject, difficult. <br />
<br />
Tone noise possible. Often run with linearzised solvers in the frequency domain.<br />
<br />
Jet noise possible. Often run with LES or DES simulations that either also resolve the sound waves or couples to a separate acoustic solver.<br />
<br />
Turbomachinery broadband noise not possible yet, or at least a great challenge.<br />
<br />
== What to trust and what not to trust ==<br />
<br />
CFD is generally quite good at predicting surface static pressure distributions. With care CFD can also be used to predict performance, total-pressure losses and blade turning. <br />
<br />
Predicting separation, stall and off-design performance can be a challenge and results with non-attached flows should be interpreted with care. <br />
<br />
Heat transfer is often very difficult to predict accurately and it is common to obtain heat-transfer coefficients that are 100% wrong or more. Validation data is critical in order to be able to trust heat transfer simulations.<br />
<br />
Transition is almost impossible to predict accurately in genereal. However, there exists models that have been tuned to predict transition and these tend to give acceptable results for cases close to the ones they were tuned for.<br />
<br />
== External links ==<br />
<br />
[http://www.qnet-cfd.net/newsletter/8th/n8_40-46.pdf QNET-CFD Best Practise Advice for Turbomachinery Internal Flows]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/User:SachinshendgeUser:Sachinshendge2005-12-02T04:48:09Z<p>Sachinshendge: </p>
<hr />
<div>Hi<br />
I,Sachin Shendge,am in the field of CFD since last two years.<br />
My email ID is sachinshendge@rediffmail.com</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-02T04:29:22Z<p>Sachinshendge: /* Conduction */</p>
<hr />
<div>== Conduction ==<br />
<br />
Conduction can be defined as the heat transfer through a substance because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
Mathematically, it can be described by using the Fourier's law: <br />
<br />
:<math>Q_{Conduction} = -k*A*\frac{dT}{dx}</math><br />
<br />
Where<br />
<br />
:<math>Q = \mbox{Heat conducted}\;[W]</math><br />
:<math>k = \mbox{Thermal conductivity of the material}\;[W/m\,K]</math><br />
:<math>A = \mbox{Cross-sectional area of the object perpendicular to heat conduction}\;[m^2]</math><br />
:<math>T = \mbox{Temperature}\;[K]</math><br />
:<math>x = \mbox{Length of the object}\;[m]</math><br />
<br />
== Convection ==<br />
<br />
<br />
<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-01T11:04:33Z<p>Sachinshendge: </p>
<hr />
<div> == Conduction ==<br />
<br />
*Conduction can be defined as the heat transfer through a substance because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
*Mathematically, it can be described by using the Fourier's law: <br />
:<math><br />
Q_{Conduction} = -k*A*dT/dX<br />
</math><br />
Where<br />
<br />
Q = Heat conducted (W)<br />
<br />
k = Thermal conductivity of the material (W/m-K)<br />
<br />
A = Cross-sectional area of the object parallel to heat conduction (m2)<br />
<br />
T = Temparature (K)<br />
<br />
X = Length of the object (m)<br />
<br />
== Convection ==<br />
<br />
<br />
<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Heat_transferHeat transfer2005-12-01T10:59:44Z<p>Sachinshendge: /* Conduction */</p>
<hr />
<div> == Conduction ==<br />
<br />
*Conduction can be defined as the heat transfer through a substance because of a temperature gradient. The rate of heat transfer by conduction between two regions of a substance is proportional to the temperature difference between them. The constant of propotionality is called thermal conductivity of the material.<br />
<br />
*Mathematically, it can be described by using the Fourier's law: <br />
:<math><br />
Q_{Conduction} = -k*A*dT/dX<br />
</math><br />
Where<br />
<br />
k = Thermal conductivity of the material (W/m-K)<br />
<br />
A = Cross-sectional area of the object parallel to heat conduction<br />
<br />
T = Temparature (K)<br />
<br />
X = Length of the object<br />
<br />
== Convection ==<br />
<br />
<br />
<br />
<br />
== Radiation ==<br />
<br />
<br />
[[category:stubs]]</div>Sachinshendgehttp://www.cfd-online.com/Wiki/Best_practice_guidelines_for_turbomachinery_CFDBest practice guidelines for turbomachinery CFD2005-12-01T10:49:13Z<p>Sachinshendge: /* Multi-stage analysis */</p>
<hr />
<div>== Introduction ==<br />
<br />
This guide is mainly aimed at axial turbomachinery. Feel free to extend it to other types of turbomachinery though. <br />
<br />
== Deciding what type of simulation to do ==<br />
<br />
=== 2D, Quasi-3D or 3D ===<br />
<br />
=== Inviscid or viscid ===<br />
<br />
=== Transient or Stationary ===<br />
<br />
== Meshing ==<br />
<br />
In general, for turbomachinery blading simulations a structured grid, especially in the boundary layers, is preferable and will most often provide superior accuracy over an unstructured grid. <br />
<br />
=== Mesh size guidelines ===<br />
<br />
In 3D a decent wall-function mesh typically has around 100,000 cells. This type of mesh size is suitable for quick design iterations where it is not essential to resolve all secondary flows and vortices. A good wall-function mesh intended to resolve secondary flows well should have at least 400,000 cells. A good low-Re mesh with resolved boundary layers typically has around 1,000,000 cells.<br />
<br />
In 2D a good wall-function mesh has around 10,000 and a good low-Re mesh with resolved boundary layers has around 30,000 cells.<br />
<br />
It is important to resolve leading and trailing edges well. Typically at least 10 cells, preferably 20 should be used around the leading and tralining edges. For very blunt and large leading edges, like those commonly found on HP turbine blades, 30 or more cells can be necessary. <br />
<br />
Cases which are difficult to converge with a steady simulation and which show tendencies of periodic vortex shedding from the trailing edge, can sometimes be "tamed" by using a coarse mesh around the trailing edge. This, of course, reduces the accuracy but can be a trick to obtain a converged solution if time and computer resouces does not allow a transient simulation to be performed.<br />
<br />
=== Boundary layer mesh ===<br />
<br />
For design iteration type of simulations where, a [[wall function]] approach is sufficient, [[y-plus | y+]] for the first cell should be somewhere between 30 and 300. For more accurate simulations with resolved boundary layers the mesh should have a y+ for the first cell which is below 1. A good rule of thumb is to use a growth ratio in the boundary layer of 1.2 - 1.25. <br />
<br />
If you are uncertain of which wall distance to mesh with you can use a [http://www.cfd-online.com/Links/tools.html#yplus y+ estimation tool] to estitmate the distance needed to obtain the desired y+.<br />
<br />
As a rule of thumb a wall-function mesh typically requires about 10 cells in the boundary layer whereas a resolved low-Re mesh requires about 40 cells in the boundary layer. <br />
<br />
== Boundary conditions ==<br />
<br />
Describe different types of boundary conditions and when they should be used:<br />
<br />
* Total pressure in, static pressure out<br />
* Absorbing boundary conditions<br />
* ...<br />
<br />
Describe how to select inlet turbulence level and length-scale <br />
<br />
== Turbulence modeling ==<br />
<br />
Selecting a suitable turbulence model for turbomachinery simulations can be a challenging task. There is no single model which is suitable for all types of simulations. Which turbulence model CFD engineers use often has as much to do with beliefs and traditions as with knowledge and facts. There are many diffrent schools. However, below follows some advices that most CFD engineers in the turbomachinery field tend to agree upon.<br />
<br />
For attached flows close to the design point a simple algebraic model like the [[Baldwin-Lomax model]] can be used. Another common choice for design-iteration type of simulations is the one-equation model by [[Spalart-Allmaras model | Spalart-Allmaras]]. The big advantage with both the [[Baldwin-Lomax model]] and the [[Spalart-Allmaras model]] over more advanced models are that they are very robust to use and rarely produce complete unphysical results.<br />
<br />
In order to accurately predict more difficult cases, like flows that are close to or even fully separated, rotating flows, flows strongly affected by secondary flows etc. it is often necessary to use a more refined turbulence model. Common choices are a two-equation models like the <math>k-\epsilon</math>. <math>k-\epsilon</math> models can give good results but this type of models need to include some form of correction to avoíd over-production of turbulent energy in regions with strong acceleration or decelleration. Typical such corrections are some form or [[realizability constraint]] or the [[Kato-Launder modification]]. Antoher common choice in turbulence model is Menter's [[SST k-omega model]] or the slightly more elaborate [[v2f model]] by Durbin.<br />
<br />
=== Near-wall treatment ===<br />
<br />
For on-design simlations without any large separated regions it is often sufficient to use a [[wall-function model]] close to the wall, preferably using some form of non-equilibrium wall-function that is sensitised to streamwise pressure gradients.<br />
<br />
For off-design simulation, or simulations involving complex secondary flows and separations, it is often necessary to use a [[low-Re model]]. There exists many low-Re models that have been used with success in turbomachinery simulations. A robust and often good choice is to use a one-equation model, like for example the [[Wolfstein model]], in the inner parts of the boundary layer. There are also several Low-Re <math>k-\epsilon</math> models that work well. Just make sure they don't suffer from the problem with overproduction of turbulent energy in regions with strong acceleration or decelleration. In the last few years Menter's low-Re <math>SST k-\omega</math> model has gained increased popularity. <br />
<br />
=== Transition prediction ===<br />
<br />
Transition refers to the process when a lamainar boundary layer becomes unstable and transitions to a turbulent boundary layer. There are two types of transition - natural transition, where inherent instabilities in the boundary layer cause the transition and by-pass transition, where convection and diffusion of turbulence from the free-stream into the boundary layer causes transition. <br />
<br />
== Numerical considerations ==<br />
<br />
== Multi-stage analysis ==<br />
<br />
Types of analysis:<br />
<br />
* Frozen rotor<br />
Frozen rotor defines the domain interface to transfer flow and thermal data across the interface between the stationary and rotating domain. <br />
* Mixing-plane<br />
* Sliding-mesh<br />
* Time-inclinded<br />
* Adamszyk stresses ...<br />
<br />
''Describe how to scale blade-sections when doing sliding-mesh computations''<br />
<br />
== Heat transfer predictions ==<br />
<br />
== Acoustics and noise ==<br />
<br />
A whole separate research subject, difficult. <br />
<br />
Tone noise possible. Often run with linearzised solvers in the frequency domain.<br />
<br />
Jet noise possible. Often run with LES or DES simulations that either also resolve the sound waves or couples to a separate acoustic solver.<br />
<br />
Turbomachinery broadband noise not possible yet, or at least a great challenge.<br />
<br />
== What to trust and what not to trust ==<br />
<br />
CFD is generally quite good at predicting surface static pressure distributions. With care CFD can also be used to predict performance, total-pressure losses and blade turning. <br />
<br />
Predicting separation, stall and off-design performance can be a challenge and results with non-attached flows should be interpreted with care. <br />
<br />
Heat transfer is often very difficult to predict accurately and it is common to obtain heat-transfer coefficients that are 100% wrong or more. Validation data is critical in order to be able to trust heat transfer simulations.<br />
<br />
Transition is almost impossible to predict accurately in genereal. However, there exists models that have been tuned to predict transition and these tend to give acceptable results for cases close to the ones they were tuned for.<br />
<br />
== External links ==<br />
<br />
[http://www.qnet-cfd.net/newsletter/8th/n8_40-46.pdf QNET-CFD Best Practise Advice for Turbomachinery Internal Flows]</div>Sachinshendge